This invention relates to an optical head, which is suitable for recording and play-back for a so-called optical disc, digital audio disc, video disc, etc.
A conventional recording and play-back head for the optical disc is so large, on the order of 40.times.40.times.30 mm.sup.3 even for that utilized in practice, and is heavy so that it prevents to reduce the volume and the weight of a whole optical device or to realize mass storage stacked optical discs. As one of the reasons therefore it is pointed out that it is difficult to realize a single axis optical system, because the light beam reflected by an optical disc is bent by means of a half prism or a polarization prism so that its optical axis is deviated by 90.degree., and detected by using a photodetector disposed therebehind.
In order to resolve this problem, a small type optical head has been proposed since several years, which head utilizes the effect that oscillation output is increased by the self coupling effect, when light is returned to the light emitting part of a semiconductor laser optical source, so-called SCOOP effect.
However, it is pointed out that the self coupling effect is a sort of instability of the oscillation phenomena of a semiconductor laser, and at present, for digital audio discs, video discs, etc., which have been commercialized recently, techniques for suppressing this effect as noise entering play-back signals or positioning signals are being developed. The self coupling effect for the semiconductor laser is an effect which can be observed in a structure of a resonator consisting of three mirrors, i.e. a reflecting surface of the optical disc added to two proper mirrors of the resonator in the semiconductor laser. Thus, due to fluctuations in the direction of the optical axis provoked by the rotation of the optical disc, the distance between the semiconductor laser and the optical disc fluctuates within a range of about 1 .mu.m and consequently the semiconductor laser has a resonator structure whose stability is very bad. Consequently, there are too many problems to be resolved to reproduce signals on the optical disc by using the SCOOP effect.